Field of the Invention
This invention relates to "free space laser communications systems" i.e. systems in which laser beams are used to communicate between two spacecraft, for example two orbiting satellites. In prior art space communication systems spacecraft is provided with means for directing a laser beam towards the other and a receiver for receiving the beam sent from the other. The beam directing means includes a laser source and projection optics, and a pointing system for steering the projected beam. The laser power is limited so each beam must be kept narrow and pointed very accurately at the other spacecraft. To achieve accurate pointing and then to maintain it in the face of relative movement of the two spacecraft, each spacecraft comprises a radiation sensitive position sensor which is activated by a part of the laser power arriving from the partner and which controls the pointing system (for example an arrangement of servo controlled mirrors). Clearly, the range of deflection of the projected beam must be sufficient to cover all possible positions of the partner as orbital motion changes the orientation between the two spacecraft. To achieve the combination of a large deflection range and great accuracy, the pointing system may comprise two cascaded pointing mechanisms i.e. a fine pointing mechanism with a narrow range but high accuracy and a coarse pointing mechanism with a larger range but poorer accuracy. Each mechanism comprise two movable mirrors, one for each deflection axis.
However, disturbances to the beam pointing may arise principally from any friction present in the pointing mechanisms, from the vibrational motion of each spacecraft and from less than perfect performance of each position sensor.
With reference to the friction effect, great improvement is possible by making use of so-called flexure suspension for the movable mirrors rather than the conventional sliding bearings. However, this is normally only feasible for the fine pointing mechanism because a flexure suspension would not be able to provide the range of movement required for the coarse mechanism.